Corrected Calcium

Calcium travels in your blood in two ways: about half sits bound to protein, almost all albumin, and the other half is free. Only the free part works, in your nerves, muscles, heart rhythm and clotting, and only that part your body guards with PTH and vitamin D. An ordinary calcium test adds the two together. As long as your albumin is normal, that makes no difference, because then the free part is a fixed fraction of the total. It goes wrong when your albumin deviates. With a liver disease, a kidney disease, malnutrition, a long-lasting inflammation or a pregnancy the albumin falls, and with every gram of albumin less the bound calcium falls and so does the total, while the free part simply stays up. Anyone who then looks only at the total reads a deficiency that is not there. Corrected calcium is the calculation that adjusts that: total calcium plus 0.025 mmol/L for every gram of albumin below forty. It works the other way too: with a high albumin from dehydration the formula subtracts a little. The result is in mmol/L, like total calcium. Healthy is about 2.15 to 2.55 mmol/L, the national band, and 2.2 to 2.6 mmol/L at the one lab that prints the calculation. Usually it is not on your report; you make it yourself with the calculator on this page.

Corrected calcium is a tool for one situation: a calcium that deviates while the albumin deviates too. In anyone who is healthy with a normal albumin, total calcium says the same and the calculation adds nothing. That is why it is usually not on your report, and why you need not miss it. Where it does count is with a low albumin, and that occurs more often than you think: in older people who eat little, in anyone who has been ill for long, with an inflammation that has been running a while, with a liver that makes less or a kidney that leaks protein. A low total calcium in that situation is usually not a deficiency, and the calculation shows that before calcium is wrongly topped up. The other way round, a normal total with a low albumin can hide a raised free calcium, and then the calculation brings to light a parathyroid problem that would otherwise have been missed. The honest limitation is that the formula dates from 1973 and assumes an average body. In people who are ill, with a markedly deviating acidity and with a high or low total protein it is regularly off, and that is why labs have become reserved about it. In case of doubt, and certainly with symptoms of tingling or cramps, free calcium is the direct measurement that needs no formula.

A low corrected calcium means the free calcium is low too, because the influence of albumin has already been calculated away. The question is then why your regulating loop is not keeping up. From common to rare: a vitamin D deficiency, in which the gut absorbs too little and PTH takes it from the bones; a kidney disease, in which the kidney no longer activates vitamin D and holds on to phosphate; and a low magnesium, without which the parathyroid glands release no PTH. Rare are parathyroid glands that work too little, usually after a neck operation. Tingling around the mouth and in the fingers and cramps are the symptoms that go with it. A high corrected calcium you take seriously, even without symptoms. Overactive parathyroid glands are the most common reason in anyone who feels well, and you recognise them by a PTH that is not low beside a high calcium. A malignancy, usually with symptoms and a low PTH, is the reason a high calcium in someone who is ill is investigated the same week. Medicines such as thiazide diuretics, lithium, too much vitamin D and calcium tablets also lift the value. Set corrected calcium beside the total calcium and the albumin it comes from, beside PTH with every deviating value, and beside vitamin D and magnesium with a low value. With symptoms or a markedly deviating albumin, free calcium is the better measure.